Bright, solid-state single-photon emitters are essential for scalable quantum photonic technology. Room-temperature switching of such emitters into and out of resonance, which is key for quantum functionality, requires reversible and wide-range tuning, but such an emitter has remained elusive. The authors report electrostatic control of a huge spectral shift for individually selected emitters in $h$-BN. Their method, based on applying an electric field via a conductive tip, is simple, yet allows for a systematic analysis of crucial properties of individual solid-state emitters. This appears to be a large step forward in integrated quantum optics.

In quantum computing, symmetry plays a central role in protecting qubits from errors. This work develops an efficient approach to creating decoherence-free subspaces for logical qubits, by optimally transferring the states of physical qubits via concatenated dynamical decoupling. This method bears a distinct superiority for many-qubit systems, owing to its polynomial speedup over previous approaches, which makes it promising for generating a higher-dimensional decoherence-free subspace to encode more protected logical qubits for fault-tolerant quantum computation.

How might one make metasurfaces “intelligent”, to offer multiple reconfigurable functions? The authors show that it can be done by embedding individually addressable tunable chips (mixed-signal integrated circuits) in each unit cell. This enables independent, continuous control of both the resistive and reactive parts of the local complex surface impedance. This allows a broader range of functionalities, among which tunable arbitrary-angle perfect absorption and tunable perfect anomalous reflection are showcased in this study. Further development could unlock other tunable functionalities, such as arbitrary wave-front shaping, space-time-modulated devices, holography, and sensing.

Relaxor ferroelectrics continue to attract great attention for applications, including electrocaloric refrigeration, but the commonly used, indirect method of electrocaloric characterization based on the Maxwell relation often leads to artifacts. This analysis of long- and short-range ferroelectric order and the electrocaloric effect in Pb${}_{0.91}$La${}_{0.06}$Zr${}_{0.8}$Ti${}_{0.2}$O${}_3$ uses both direct and indirect characterizations, to clarify the applicability of the Maxwell relation to the physics of the relaxor, and the origin of discrepancies. It also suggests how to design relaxors with large electrocaloric effect by exploiting the transitions between polar nanoregions.

Superconducting qubits are a promising platform for quantum computing, but measurements of such circuits rely on the use of ferrite circulators, which are difficult to miniaturize or make lossless. Replacing those circulators with on-chip superconducting ones has become a major research thrust, but so far most of these replacements have been narrow-band. Thus the authors design an on-chip circulator that combines low-loss circulation with instantaneous bandwidth an octave wide. Such a device could enable the multiplexed readout of hundreds of qubits, facilitating the scale-up that is currently a major hurdle in quantum information processing with superconducting circuits.

High-fidelity readout of semiconductor-based qubits (in particular, Majorana qubits of the future) could be accomplished by dispersive readout, as already used with superconducting qubits. So far, though, the dispersive signals have been small, and the required integration times much longer than typical qubit coherence times. This work shows that signal amplitude can be vastly increased by strongly coupling a sensing gate to a readout quantum dot. This coupling allows readout of this system in the microsecond regime, which is on par with the state of the art for other qubits. Here the chief limiting factor of the signal-to-noise ratio is tunnel coupling.

To study topological phases, break out your soldering iron… The valley Chern effect is interesting because robust interfacial modes can be created in a honeycomb lattice by a simple breaking of the system’s inversion symmetry. The effect has been emulated on a number of classical platforms, including mechanical and acoustic systems, but not on an electromagnetic one. This work proposes a modular setup of inductively coupled $LC$ resonators and shows how it can reproduce the valley Chern effect. Using realistic values for the electronic components, these circuits display extremely high $Q$ factors, making them ideal for exploring topological phenomena.

Calculating the energy spectra of molecules is a key computational problem, and one where a quantum computer can shine. For present-day quantum computers, short quantum algorithms that finish within the coherence time of the system must be designed. Thus the authors present a set of gates tailored to the problem at hand, which can be directly implemented in hardware. Experiments show that exchange-type gates that conserve the number of excitations are ideally suited for calculations in quantum chemistry. The team determines the energy spectrum of molecular hydrogen using a variational quantum eigensolver, plus a method from computational chemistry to compute the excited states.

Bright, solid-state single-photon emitters are essential for scalable quantum photonic technology. Room-temperature switching of such emitters into and out of resonance, which is key for quantum functionality, requires reversible and wide-range tuning, but such an emitter has remained elusive. The authors report electrostatic control of a huge spectral shift for individually selected emitters in $h$-BN. Their method, based on applying an electric field via a conductive tip, is simple, yet allows for a systematic analysis of crucial properties of individual solid-state emitters. This appears to be a large step forward in integrated quantum optics.

In industry, magnetic resonance in porous media is widely employed to investigate pore-fluid behavior, including relative pore size. The full potential of this technique has not been realized, though, due in part to neglect of nonground eigenstates. The authors show that in porous media nonground eigenvalues do contribute to the relaxation of initially homogeneous magnetization, by longitudinal and transverse processes, permitting the estimation of $absolute$ pore size. Their method is able to transform relaxation lifetimes to absolute pore sizes, without calibration measurements, which means a large body of existing data can be reprocessed to yield the confinement sizes of materials.

For commercially viable nonvolatile magnetoresistive random-access memory (MRAM), devices are needed in the 10–20-nm size range, where thermal and dynamical effects can be significantly different from those in the bulk. Using atomistic spin dynamics, the authors study nanoscale samples of Co-Fe-B/MgO, and attribute the empirical thickness dependence of Gilbert damping to an enhanced interfacial contribution. They also find a stochastic contribution to the damping parameter, leading to a finite-size effect by which the dynamical constants are time-dependent. These results have important consequences for device switching that must be reckoned in future design and optimization.

Ferroelectric materials attract attention as potential solid-state thermal switches, due to various phenomena near their solid-state phase transition. In principle these phenomena could also be triggered by an external electric field, though recent experiments on thin films have shown only weak effects. The authors explore this issue by solving the linearized Boltzmann equation with first-principles force constants, to study the thermal conductivity of PbTiO${}_3$ as a function of electric field. The results emphasize the need for large-domain samples and lower temperatures, to have the best chance at large voltage-controlled thermal switching in this class of materials.

Modern particle accelerators utilize superconducting cavities cooled by superfluid helium, the maximum accelerating field being limited by cavity quenching, caused by heating from tiny surface defects. Locating these quench spots and subsequently removing the defects can improve the maximum accelerating field, for better accelerator performance and cost reduction. This article reports an approach for locating quench spots that significantly advances the state of the art. A proof-of-concept experiment also uncovers important physics for solving a decades-long puzzle underlying the operation of a widely adopted cavity diagnostic.

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency $f_e$ is continuously tunable via changing the bias current and the bath temperature, but at low bias $f_e$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.

The ideality factor of a solar cell, derived from the dependence of its open-circuit voltage $V_{\text{OC}}$ on light intensity, has historically been used to identify the dominant mechanism of charge-carrier recombination in a device. Applying this technique to perovskite solar cells (PSCs) is problematic, though, since $V_{\text{OC}}$ values change with time, due to the presence of slowly mobile ionic charge in the perovskite layer. Using both transient optoelectronic measurements and drift-diffusion simulations, the authors identify the dominant recombination mechanism in PSCs from the $transient$ ideality factor, following device preconditioning with a forward bias.

Two-dimensional holographic leaky wave radiators are important tools for electromagnetic beam forming. However, the lack of reasonable bandwidth in operating frequency has always been a major drawback of such structures. This study proposes a parabolic surface-wave reflector to yield wide-band radiators. Using a suitably designed metasurface and the holography principle, this technique directs all of the energy generated by a cylindrical wave launcher into the beam-forming process as the forward leaky mode. Such a planar holographic antenna is capable of beam scanning by frequency variation.

The photovoltaic effect seen in ferroelectric oxides is gaining interest for next-generation solar cells, as these materials do not suffer from the same issues regarding carrier recombination and band gap as do the commonly employed semiconductors. The authors demonstrate a large photovoltaic response in a Pb-free mixed titanate-zirconate system by electrically tuning the material’s symmetry-breaking structural transformation, and correlate the effect to the preferred structural symmetry via band-structure calculations. These results give welcome insight for engineering ferroelectric oxides for photovoltaic applications.

Dynamic helicity control with metasurfaces is essential for chirality-sensitive spectroscopy, data transmission, and sensitive measurements, but is challenging, as it requires deep modulation of light’s polarization states. To conquer the challenge, this study leverages metallic checkerboards, the electromagnetic characteristics of which critically depend on the connection states. The authors establish a general guiding principle for realizing helicity switching with such structures, and implement a dynamic terahertz quarter-wave plate to that end. This approach paves the way to the effective use of light’s spin degree of freedom.

The familiar “Newton’s cradle” demonstrates conservation of momentum and energy using a series of swinging identical spheres. Reaching beyond desktop toys, the authors explore a photonic analog of Newton’s cradle, and show its utility for energy transport in optical quantum information processing. In a chain of 21 coupled sites on a photonic chip, long-range interactions are mediated as single-photon excitations are transferred between pairs of remote sites, via simultaneous control of weak and strong couplings. This approach offers flexible Hamiltonian engineering beyond geometric limitations, enabling on-demand design and construction of integrated networks for quantum simulation.

Strained-silicon technologies exploit mechanical stress to improve the speed and gain of integrated circuits. Here impedance spectroscopy reveals that when mechanically bent, the electrical properties of fully depleted silicon-on-insulator exhibit giant, anomalous changes far larger than those already used in strained-silicon technology. The effect does not occur under steady-state conditions, suggesting that it is due to the stress dependence of charge-trapping rates at deep centers associated with crystal defects. This insight sheds light on giant piezoresistance in silicon nanostructures, and is a step toward strained-defect technologies for any device based on deep centers.

Thanks to their tunable dipole moment and spin-orbit coupling, type-II quantum dots based on antimony could be relevant for quantum technologies that so far have only used type-I nanostructures. Thus the authors study the optical properties of InAs/Ga(As,Sb) quantum dots embedded in a $p-i-n$ diode. Vertical electric and magnetic fields induce Aharonov-Bohm oscillations, which are related to a change in topology of the hole wavefunction. Due to the spin-orbit coupling in the Sb-bearing layer, the pronounced changes in orbital confinement also bring a modulation of the hole’s spin properties. These further degrees of freedom could be a means to improved spin-photon interfaces.

Photostimulation of neurons by photoactive substrates might revolutionize the treatment of neurological disorders like retinal degeneration, if high-level control of stimulation mechanisms can be achieved. So far these mechanisms have been governed by changing the surface materials interfacing with cells. This study, on the other hand, controls the processes by changing the inner intermediate layer, through proper band engineering. Incorporating appropriate nanoscale materials into organic photovoltaic biointerfaces can enhance their operation at low light intensity, opening a way toward superior neural prosthesis.

Spiral phase contrast (SPC) imaging is an important technique in edge detection. For infrared wavelengths, though, typical charge-coupled-device detectors are inefficient, slow, and noisy; to exploit them, one should instead work in the visible part of the spectrum. Here up-conversion SPC imaging is realized, based on sum-frequency generation, which also has the advantage of enhancing the field of view. This versatile technique is quite promising for $e.g.$ reagent-free biological imaging, pattern recognition, and up-conversion edge detection.

Quantum decoherence due to charge fluctuations is still a problem in many device architectures for quantum computing or metrology, but it can be suppressed by shunting the circuit with a high microwave impedance, known as a $superinductance$. This study uses the kinetic inductance of a nanowire of disordered superconductor to demonstrate a low-loss, small-capacitance superinductor, in a simpler approach than the traditionally used Josephson-junction arrays. Such nanowire superinductors should have an impact on engineering long-lived superconducting qubits, and quantum-coherent experiments with mesoscopic systems.

The laminar-turbulent transition in hypersonic flows is associated with the acoustic-wave-like Mack second mode. Understanding the physics and suppression of this mode is not only of basic scientific interest, but is also essential to future high-speed transportation. This study reveals that, counterintuitively, an almost nondissipative boundary with near-zero surface acoustic impedance can suppress this mode, and performs even better than conventional dissipation-based ultrasonic absorptive coatings. A practical realization, based on an acoustic metasurface, is furthermore presented, pointing toward full control of the hypersonic boundary-layer transition. Full speed ahead!

Adjacent layers of magnetically hard and soft materials are of both fundamental and technological importance, yet systematic experimental studies of how to control the interlayer exchange coupling (IEC), and how the IEC affects the magnetization dynamics in each layer, have been limited. In this study, broadband ferromagnetic resonance up to high temperatures is used to study IEC in a system with a hard Fe-Pt layer and a soft Fe-Co layer. The IEC of the soft layer strongly depends on the choice of soft material, its thickness, and temperature.

Dissipative-Kerr-soliton (DKS) frequency combs in nonlinear microresonators, which offer small footprints and low power consumption, can be used for optical metrology and communication. Ultrabroadband DKS combs at near-infrared frequencies remain largely unexplored, though, for lack of suitable group-velocity dispersion, reliable fabrication, and a robust method for soliton generation. Using microresonators from a commercial foundry that yield the right dispersion, the authors reliably produce stable DKSs by ultrafast pump-frequency modulation. With tuning, this work can directly connect the C and L telecommunication bands to the electronic transitions used for optical atomic clocks.

Quantum light-matter interaction underpins many optical technologies in science, medicine, and engineering, such as spectroscopic characterization, trace chemical detection, single-photon sources, and radiative heat transfer. Research here is driven by optical nanostructures, with great demand for efficient simulation tools offering physical insight. The authors develop a modal expansion of the Green’s tensor, a key tool in this context, generalized to lossy or open systems. Their simple yet powerful treatment, based on an unorthodox set of modes, provides numerous practical and fundamental advantages, bypassing the difficulties inherent to more conventional approaches.

This study presents experimental demonstrations of an innovative swimming microrobot based on elastically linked magnets. Many scientists assume that microrobots obey special physics because fluid friction is proportionately large, but the authors show that this is not always true. This is important to know, because the physics due to high fluid friction can prevent liquids from mixing in $e.g.$ medical tests that are being developed to diagnose several diseases simultaneously. A special feature of the robot here is that it can act alone or as a building block to join forces with others, which could enable teams of robots to be programmed to tackle specific problems.

The development of practical silicon spintronic devices relies on a comprehensive understanding of the spin-transport properties of the silicon channel. When the drift electric field in the channel is small, spin transport is described well by spin diffusion alone, but at larger fields spin drift starts to play a role. The authors present a simple, general, accurate method to quantify spin drift in nonlocal spin-transport devices, showing that in heavily doped Si, drift electric fields of ±400 V/cm increase or reduce the spin-transport length by about a factor of two. Such quantification of spin drift is important for correct assessment of device characteristics and performance.

Thermal management, with a twist: The function of a traditional thermal metamaterial is predetermined once the structure is fixed, and it is difficult to tune thermal conductivities $in$ $situ$. This work proposes a doublet thermal metadevice that enables multiple functions by mechanically rotating identical singlet configurations. The doublet device consists of two conductive materials in a stacked layout with a sort of curved chessboard structure, so that its function can be reconfigured by adjusting the angular displacement of the moving parts. Simulations show that a single device can serve as a thermal cloak, concentrator, or rotator, or can appear transparent.

Spectrally stable quantum emitters, robust spin-photon interfaces that are insensitive to stray electric fields, are great for quantum information processing. Here quantum defects with inversion symmetry are seen as the ultimate solution, but the authors show that inversion symmetry is $not$ a prerequisite for weak coupling to electric fields during optical excitation. Rather, the same spatial localization of ground and excited wave functions of defect states is sufficient. Calculations of a silicon-vacancy center in SiC reveal that it is such a nearly ideal quantum defect. These findings expand the search for ideal quantum emitters in compound semiconductors.

In quantum computing, symmetry plays a central role in protecting qubits from errors. This work develops an efficient approach to creating decoherence-free subspaces for logical qubits, by optimally transferring the states of physical qubits via concatenated dynamical decoupling. This method bears a distinct superiority for many-qubit systems, owing to its polynomial speedup over previous approaches, which makes it promising for generating a higher-dimensional decoherence-free subspace to encode more protected logical qubits for fault-tolerant quantum computation.

How might one make metasurfaces “intelligent”, to offer multiple reconfigurable functions? The authors show that it can be done by embedding individually addressable tunable chips (mixed-signal integrated circuits) in each unit cell. This enables independent, continuous control of both the resistive and reactive parts of the local complex surface impedance. This allows a broader range of functionalities, among which tunable arbitrary-angle perfect absorption and tunable perfect anomalous reflection are showcased in this study. Further development could unlock other tunable functionalities, such as arbitrary wave-front shaping, space-time-modulated devices, holography, and sensing.

The separation of chiral objects with opposite handedness is an important scientific and technological challenge, especially in pharmaceutical research and production. Although established techniques operate at industrial scale, improvements are welcome, especially toward the development of compound-insensitive processes. This study reports experiments to resolve a racemic mixture of chiral particles by purely optical means, which also lead to numerical exploration of the cooperative effects of hydrodynamic interactions mediated by the viscous fluid surrounding the particles, which can speed up the chiral sorting process.

Research into hybrid quantum systems featuring both conventional and topological qubits is of keen interest for quantum information processing, and a key challenge is to realize a coherent interface between such qubits of different nature. The authors find that a topological qubit can be interfaced to a single nitrogen-vacancy center via a magnetized torsional cantilever. Topology-torsion couplings are induced by the magneto-Josephson effect, while spin-torsion couplings are realized by the exquisite preparation of dressed spin states. These coherent interactions can reach the strong-coupling regime, and enable a mechanically-dark-state protocol for quantum state conversion.

Most models of intracavity oscillators based on nonlinear optical frequency conversion expand resonating fields in a set of cavity normal modes, the so-called Slater normal-mode expansion (NME) method. Such an approach normally yields slow variation of field amplitude, on the scale of the cavity’s round-trip time. To study faster amplitude oscillations, the authors derive the time-delay (TD) model of intracavity nonlinear optical interaction. Their model should foster a deeper understanding of the dynamics of intracavity optical parametric oscillators and difference-frequency generators, devices that are almost ideal for high-resolution spectroscopy in the midinfrared spectral window.

The interaction between light and matter is key in both basic science and technology. The strength of the light-magnet interaction, however, is very weak, which has been the main challenge in optically probing or controlling magnetism. This study shows that the magnetic field pulses generated by plasmonic nanoantennas can effectively manipulate magnetism, on a very short time scale. Here the excited magnetization undergoes coherent oscillation at ~100 THz, followed by the emission of propagating spin waves with frequencies of a few GHz. These results reveal the strong potential of plasmonic nanoantennas in spintronics.

Floquet topological insulators have inspired analogs in photonics, optics, and acoustics, in which nonreciprocal wave propagation in time-modulated materials is achieved by breaking time-reversal symmetry. This paper investigates a mechanical-wave analog of Thouless pumping and the quantum Hall effect respectively in one- and two-dimensional periodically time-modulated materials. These systems feature robust edge states that are immune to back-scattering by sharp corners, defects, and nonuniform phases. The work sheds light on the design and fabrication of quantum-Hall-type elastic topological insulators, to control elastic waves for broadband one-way transport in practical applications.

The realization of ultranarrow-band perfect absorption using tiny plasmonic structures can benefit the integration of high-$Q$ light absorbers, although it is still challenging. Ultranarrow absorption traditionally relies on lattice resonances, while small absorbers typically support broad localized resonances. Here the authors propose combining high-order plasmons beyond dipolar ones in localized resonances and optical-gain media. The absorbers they design are excellent index sensors that improve sensing of minute amounts of analytes. These results should stimulate researchers to customize absorption in plasmonic systems by leveraging elaborate mode structures and optical gain.

Field mapping of the acoustic radiation force and acoustic streaming is important for the design and calibration of acoustic tweezers, but we lack a method to disentangle and obtain accurate pictures of these fields. This study proposes a technique for multiradius microparticle image velocimetry, to solve this problem. The method requires no special assumptions about the driving acoustic field, and motorized scanning is unnecessary. This approach should impact various technologies for particle manipulation, including those exploiting acoustic, optical, dielectrophoretic, and magnetic forces.

Relaxor ferroelectrics continue to attract great attention for applications, including electrocaloric refrigeration, but the commonly used, indirect method of electrocaloric characterization based on the Maxwell relation often leads to artifacts. This analysis of long- and short-range ferroelectric order and the electrocaloric effect in Pb${}_{0.91}$La${}_{0.06}$Zr${}_{0.8}$Ti${}_{0.2}$O${}_3$ uses both direct and indirect characterizations, to clarify the applicability of the Maxwell relation to the physics of the relaxor, and the origin of discrepancies. It also suggests how to design relaxors with large electrocaloric effect by exploiting the transitions between polar nanoregions.

In the quest for what might replace conventional electronics, the emerging field of $valleytronics$ (capitalizing on local extrema in a semiconductor’s band structure for information processing) is a contender. Here the valley filter and valley valve are two key elements, and it is important to combine them in a unit. The authors present a simple, viable model for such a unit, based on bilayer graphene. Device operation is based on valley-contrasting transmission, due to the valley Zeeman effect and a band gap tunable by gate voltage, by which nearly perfect filtering and very high valve efficiency are achieved. This system could yield a reversible NAND gate, for example.

Optical magnetometers are important in a myriad of applications requiring highly sensitive measurements of magnetic fields. However, the presence of technical noise often degrades their performance severely at low Fourier frequencies, thereby preventing their use when high stability over long time scales is needed. This study demonstrates an ultrastable optical magnetometer that exhibits high sensitivity across $eight$ orders of magnitude in Fourier frequency. The contributions of a multitude of noise sources, both fundamental and technical, are investigated. The measured noise floor is primarily limited by magnetic field fluctuations, not the sensor itself.

Ultracold neutral atoms offer a promising route toward scalable quantum computing—a route that is unfortunately hindered by Doppler dephasing, a major stumbling block that spoils the fidelity of entangling gates. This study uses a theory based on quantum interference to show that it is possible to significantly suppress Doppler dephasing, allowing a high-fidelity entangling gate even with present-day technology. The interference-induced entanglement described here not only lays a foundation for such neutral-atom gates, but also sheds light on quantum information science involving other physical systems.

Silicon doped with Se${}^{+}$ is particularly compelling as a spin-photon interface, because it could be the basis of all-silicon, hybrid spin-photon quantum information technology. This study pins down the most critical spin-photon properties of Si:Se${}^{+}$: the transition dipole moment of the spin-dependent optical transition, the radiative efficiency of the first excited state, and the zero-phonon emission fraction. The authors also measure a long $T_1$ spin lifetime in Earth’s magnetic field of over 4.6 hours. Taken together, these favorable results indicate that an integrated quantum optoelectronic platform based on Si:Se+ is well within reach of current integrated photonic capabilities.

Can industrial waste heat be used to our advantage when developing environmentally friendly energy concepts, such as reverse electrodialysis (RED)? The effects of thermal driving forces in such systems have not been investigated much, despite an abundance of low-grade heat sources. The authors show that heating the low-salinity input to a RED plant can increase the electromotive force by up to 1.3% per degree Celsius of temperature difference between the low- and high-salinity inputs. This work motivates further research on the mechanisms at play, so that emerging technologies can be optimized for multipurpose applications, such as a combined RED plant and waste-heat harvester.

In advancing the performance of metamaterials, key features nowadays are tunability and a wide range of operating frequencies. Here a helical acoustic metasurface, capable of providing a modulated wavefront of reflected sound with continuously tunable broadband operation, is demonstrated. The authors’ matched screw-and-nut mechanism can lead to pragmatic applications of metasurfaces for three-dimensional control of sound. This concept for continuous tuning is an important step for achieving broadband, multifunctional acoustic metasurfaces, and may inspire designs featuring curved tunable, active programmable, or randomly coding metasurfaces.

Materials with high spin polarization, such as Heusler compounds, are required for efficient spintronics. The authors propose an approach to probe the transport spin polarization at interfaces, using the recently discovered unidirectional spin Hall magnetoresistance. They show that insertion of thin Ag(001) layers clearly increases the interfacial spin polarization of the Heusler compound Co${}_2$MnSi, which is crucial for giant-magnetoresistance devices.

Unlike familiar rooftop solar cells, thermophotovoltaics (TPVs) harvest photons at infrared wavelengths, and can scavenge energy effectively from sources other than the sun. With the advantage of easy fabrication, a near-field TPV system based on Schottky junctions has drawn much attention lately, focused mostly on the photocurrent generated in the metal side. Here a detailed model considering photocurrent generation in the $semiconductor$ side as well shows that this contribution should not be neglected, and that further enhancement of photocurrent can be achieved by reabsorbing photon energy in the metal with the help of a backside reflector.

According to recent predictions, a microscale ferromagnetic needle will precess under the influence of a small applied torque, if sufficiently isolated from the environment. In fact, such a system could be used to measure torques with a sensitivity far surpassing the standard quantum limit for a free ensemble of spins. To observe precession and carry out ultrasensitive torque measurements, the ferromagnetic needle requires nearly frictionless suspension. Here crucial initial steps are realized by levitating a micrometer-scale ferromagnetic particle above superconducting Nb and investigating its dynamics, including the role of trapped flux.

Metamaterials have generated considerable research interest in manipulating the polarization state of light, but many proposals suffer from narrow bandwidth or low transmission efficiency. Here researchers show that a classical multiple-beam interference mechanism can be used to modulate the phase dispersion of transmitted waves in metamaterials, which motivates a general strategy to achieve a broadband wave plate by independently controlling the phase dispersion for the two orthogonal polarizations. These findings can stimulate the production of high-performance broadband optical devices based on various metamaterials in different frequency domains, to impact applications in photonics.

Controlling the conversion of light’s spin angular momentum to orbital angular momentum (OAM) is crucial for applications, including optical systems and wireless communication. In this regard, metasurfaces are often limited by the difficulty of producing independent spin-to-OAM conversions. This study uses a reflective, dual-helicity decoupled coding metasurface to realize completely independent control of OAM vortices for two orthogonal helicities, achieving a free combination of distinctive OAM topological charges, arbitrary helicity, anomalous scattering, and complex spatial beam editing. These results could help to integrate versatile functionalities for advanced compact systems.

Manufacturing whispering-gallery-mode resonators from isotopically pure ${}^{28}$Si is promising for both solid-state clocks and qubits, due to the material’s extremely narrow spin linewidths and low microwave losses. However, machining procedures introduce losses into these systems, spoiling performance. This study uses Raman spectroscopy and post-machining processes to restore the high microwave $Q$-factors of these resonators. Conclusions are also drawn concerning the origin of the microwave losses and how they might be further reduced in the future, to attain extremely low-loss systems for $e.g.$ hybrid quantum information processing.

High-electron-mobility transistors (HEMTs) made of wide-band-gap semiconductors have great potential for power electronics and radio-frequency applications. Coupling first-principles calculations with device simulations enables cost-effective semiconductor research and development, including materials exploration and device design. The authors report innovative, comprehensive calculations of electronic transport in wurtzite GaN and AlN and in an (Al,Ga)N/GaN HEMT. The hot-electron energy distributions in the simulated HEMTs can be used to determine the related device degradation, and suggest opportunities for improved designs.

Skyrmions in frustrated magnets have multiple degrees of freedom that could be used to carry information in spintronic devices. In developing applications, it is important to understand the evolution in time of these twirly magnetic textures, but the dynamics of frustrated skyrmions remain elusive, especially for systems with more than a single layer. Here the authors explore the dynamics of frustrated skyrmions induced by spin currents in an antiferromagnetically exchange-coupled bilayer. The results are expected to inform the design and development of skyrmionic devices based on frustrated magnets.

Aiming for efficient thermoelectric energy conversion, the authors propose a mechanism that allows for considerable negative and positive Seebeck coefficients in a single material system, the 3:1 LaNiO${}_3$–LaAlO${}_3$ (001) superlattice. The key idea is to tune the position of emergent transport resonances relative to the Fermi energy, exploiting moderate epitaxial strain as the control parameter. This strategy furthers the intensely pursued orbital-polarization design of oxide heterostructures, in this case for important applications in energy harvesting.

Superconducting qubits are a promising platform for quantum computing, but measurements of such circuits rely on the use of ferrite circulators, which are difficult to miniaturize or make lossless. Replacing those circulators with on-chip superconducting ones has become a major research thrust, but so far most of these replacements have been narrow-band. Thus the authors design an on-chip circulator that combines low-loss circulation with instantaneous bandwidth an octave wide. Such a device could enable the multiplexed readout of hundreds of qubits, facilitating the scale-up that is currently a major hurdle in quantum information processing with superconducting circuits.

In recent years, parity-time ($PT$) symmetry has been explored in several areas of physics, including quantum mechanics, optics, and acoustics. Now medicine can reap the benefits: Utilizing anisotropic transmission resonances supported by a $PT$-symmetric structure, this study develops a noninvasive glucose sensor operating in the microwave band. Analytical results and full-wave simulations indicate that the resonance shift can be made fairly linear with respect to variations in glucose concentration, overcoming traditional limitations of passive-sensing schemes. Notably, this detection mechanism offers the possibility of precise sensors for other applications as well.

Phase-only spatial light modulators (SLMs) are very useful and versatile components for various wavefront-shaping applications. The phase calibration of such a device is fundamental for reliable use, but often depends on building separate interferometric setups. This work presents an alternative that instead depends on intensity variations of a phase-modulated beam at the focus of a microscope objective. The technique is general and robust, and provides an $in$ $situ$ approach to calibrate SLMs already implemented in microscopy setups, without modifying the original alignment.

Random distributed feedback (DFB) fiber lasers have shown potential for applications in telecommunication, sensing, nonlinear optics, and imaging, due to attractive features such as low cost, simple technology, and modeless behavior. Cylindrical vector beams (CVBs) have also received much attention, for high-resolution imaging, optical trapping, telecommunication, and sensing. Little work has been done to combine these systems, but this study demonstrates a random DFB fiber laser successfully generating CVBs. Multimode behavior indicates low temporal coherence of the output, as intended, and laser speckle is reduced by design.

Improving the performance of promising photocatalytic BiVO${}_4$/$g$-C${}_3$N${}_4$ heterostructures requires in-depth understanding and fine tuning of their near-edge electronic properties. Here researchers investigate the band-edge states of the system using a hybrid-functional approach, and identify the desired type-II band alignment, with band-edge states, band gap, and band alignment readily tunable via an applied external field. The contrasting responses of the fundamental and optical gaps suggest an avenue for optimizing the optical absorption and carrier-separation dynamics of these heterostructures for photoelectrochemical applications.

Silicon may be king of semiconductors, but for optoelectronic devices it has the drawbacks of low photoabsorption and emission efficiency, originating entirely from its indirect band gap. However, the authors’ realistic first-principles calculations indicate that direct wave-vector excitation (interband transitions between different wave numbers, without phonon assistance) can be induced simply by irradiating in the optical near field, rather than the far field. This observation would seem to suddenly remove the key stumbling block to leveraging mature silicon technologies for advances in optoelectronics.

Metamaterial-inspired diffraction gratings, or $metagratings$, have recently demonstrated superb efficiency in wavefront manipulation. Unfortunately, the absence of a systematic design procedure has held back the development of complex structures operating in different parts of the electromagnetic spectrum. The authors present a simulation-based approach for constructing metagratings in a “unit cell by unit cell” manner, and offer designs for electrical and magnetic metagratings operating in the microwave and infrared domains. These results will surely promote progress in metagrating applications.

Passively mode-locked lasers have become a topic of substantial research, as they are efficient sources of ultrashort optical pulses and essential to applications such as multiphoton microscopy and dual-comb spectroscopy. Here external cavity geometry has a great influence on performance, though, and also can lead to detrimental multipulse emission. The authors use a delay differential equation model, derived for these types of lasers and accounting for the external cavity geometry, to better understand the emergence of detrimental dynamics. The results are expected to further improve the performance of these systems.

The polarization of electromagnetic waves, in particular of light and radio waves, is nowadays an effective tool to control the transfer of energy and information. The authors’ work shows a means to discriminate microwaves of the same frequency and traveling in the same direction, but bearing different polarizations. They use their approach to specially design distributions of the refractive-index gradient in thin, heterogeneous dielectric layers. Their flexible, exactly solvable mathematical model of polarization-based gradient wave filters may become useful for counterparts in optics, electromagnetism, and matter-wave physics as well.

Devices based on surface acoustic waves (SAWs) are employed in many engineering domains, for example, as delay lines and filters in electronics, and as particle sensors for lab-on-chip systems. All of these applications would benefit from tunable devices that can vary the frequency and speed of the traveling signal. The authors present a tunable metamaterial platform to control SAWs, exploiting contact resonances modulated with permanent magnets to generate and shift SAW band gaps. The concept could be miniaturized for application in telecommunication and sensors.

The charge trapping that occurs at a semiconductor/dielectric junction is an important issue in semiconductor physics, $e.g.$ for MOSFETs. Unfortunately, first-principles research here is complicated by many technical difficulties. This study provides a general, integrated framework, including structure construction, GPU-accelerated DFT calculations, and Marcus theory, to study the charge trapping from crystalline Si to defects in amorphous SiO${}_2$. The results shed light on critical questions concerning what dominates the trapping rate, how hopping decays with distance, and how large fluctuations are induced by the amorphous nature of the dielectric.

The automated trapping of a “liquid marble” (a nonwetting droplet coated in a hydrophobic shell of micro- or nanoparticles) is interesting and important for microfluidic applications in $e.g.$ three-dimensional cell culture, microbioreactors, and sensitive detection of water or air pollution, yet remains a technical challenge. The authors use dielectrophoresis to trap a floating liquid marble with an electric field under various experimental conditions. This method should have an impact on engineering solutions for controlled manipulation of liquid marbles, with no need for special coatings.

In designing spintronic devices, control of spin-current transport at the interface of a magnetic material and a normal metal is key. In general, spin current in metallic materials is limited to the nanometer scale by the spin diffusion length. Using an ionic-gating technique, this study experimentally demonstrates control of the spin current due to spin pumping in a yttrium-iron-garnet/Pt bilayer using a gate voltage. This result helps to better understand spin transfer at magnetic-insulator/heavy-metal interfaces, and therefore enables improved spintronic devices.

High-fidelity readout of semiconductor-based qubits (in particular, Majorana qubits of the future) could be accomplished by dispersive readout, as already used with superconducting qubits. So far, though, the dispersive signals have been small, and the required integration times much longer than typical qubit coherence times. This work shows that signal amplitude can be vastly increased by strongly coupling a sensing gate to a readout quantum dot. This coupling allows readout of this system in the microsecond regime, which is on par with the state of the art for other qubits. Here the chief limiting factor of the signal-to-noise ratio is tunnel coupling.

Measuring small currents and mapping nonlinear electrical properties are key to characterizing modern devices and investigating physics at the nanoscale. Here the high capacitance and low conductance of nanojunctions limit measurement speed. The authors introduce a multifrequency technique to separate Galvanic and displacement currents, and to compensate for parasitic contributions, achieving a speedup of four orders of magnitude when measuring current-voltage characteristics with a conductive atomic force microscope. This flexible approach for fast, high-resolution electrical characterization in nanotechnology is also directly applicable to scanning tunneling microscopy.

Spin relaxation times are critical parameters in quantum information processing. Recent experiments with single-spin qubits in silicon have achieved qubit control via electric dipole spin resonance, in the presence of gradient magnetic fields generated by on-chip micromagnets. These studies have reported drastically reduced spin-relaxation times compared to those in traditional devices. Studying the magnetic field dependence of the relaxation rate, the authors find that the synthetic spin-orbit field introduces additional relaxation mechanisms, which they quantify. This insight will help to optimize the compromise between fast qubit control and enhanced spin relaxation.

Acoustically driven micro- and nanomachines seem promising, by virtue of their simplicity and universality, but effective methods to realize controllable high-speed rotation at these length scales are still lacking. This study investigates an actuation mechanism based on local enhanced microstreaming in a low-frequency acoustic field, and proposes a high-speed acoustic rotary microsystem. Using this simple, controllable methodology, a polystyrene rotor 10 $\mu$m in diameter can be spun at 5000 rpm, which should be useful in lab-on-a-chip applications.

The ability to sequentially join individual circuit elements is vital for the development of more complex magnon-based computers, which could overcome the limits imposed by the looming end of Moore’s Law. Efforts are currently impeded because there is no single method for encoding data into magnonic signals, leaving some devices inherently incompatible with others. This study addresses the problem by presenting a unified language to describe the various encoding techniques, and by providing a practical method for converting between them. Using a hybrid of the established encoding techniques, additional circuit elements can be developed for innovative logic operations.

Correlating surface features and the resultant evolution of optical properties is important in the study and application of two-dimensional (2D) materials like transition-metal dichalcogenides. The authors measure photoinduced forces in atomically thin layers of MoS${}_2$ and WS${}_2$ using a modified atomic force microscope, enabling their visualization with high contrast. A spectral-force retrieval technique enables evaluation of the distinct excitonic peaks of the two structures, which resemble the excitonic features of far-field absorption. Spectral signatures of defects and edges are also observed, opening the way toward nondestructive, high-resolution defect imaging of 2D materials.

The lateral wet oxidation of thin layers of Al-rich semiconductors is of prime importance for the fabrication of electrically injected vertical-cavity lasers, and in many areas of nanophotonics. The authors introduce a reaction-diffusion model of oxidation-front propagation based on the chemical oxidation processes, one that includes the effects of anisotropies and applies to any (possibly nonconvex) initial geometry. Numerical simulations show excellent agreement with experiment. This method is general and opens the way to the fine control of wet-oxidation fronts, which has numerous practical applications in terms of device creation.

$Self-parametric$ $amplification$ is an unusual nonlinear process in which the spectrum of an optical field narrows upon propagation in a normal-dispersion optical fiber—in sharp contrast to the conventional spectral $broadening$. This nonlinear effect is conducive to developing lasers with high spectral brightness, which, however, inherently contradicts the bound condition. This article shows that the dilemma can be overcome via the self- adaptation of a dissipative soliton laser with advanced dispersion engineering. The critical laser dynamics here are relevant to the Fermi-Pasta-Ulam paradox and breathing soliton dynamics, and the results shed some light on those phenomena as well.

Among advanced paradigms in the physics of computing, adiabatic quantum-flux-parametron (AQFP) logic is interesting, in that it can easily introduce stochastic processes by exploiting naturally occurring thermal fluctuations. This study proposes using AQFP logic to implement stochastic-local-search solvers. Experiments show that AQFP circuits can find solutions to simple logical constraint-satisfaction problems when moderate thermal fluctuations are applied. This result indicates the possibility of superconducting digital circuits dedicated to solving combinatorial optimization problems.

Spin-orbit torque is widely studied as a switching mechanism for magnetic memory and spin logic devices. In typical device configurations, a lot of current flows through the shunting path of the ferromagnetic metal, significantly increasing energy consumption. This work demonstrates that spin-torque transfer can be realized across an antiferromagnetic insulator: By applying current in the heavy-metal layer, one can achieve magnetic switching in the magnetic free layer, despite the antiferromagnetic-insulator layer in between. Thus electrically isolated reading and writing paths in spin-orbit-torque devices, for energy-thrifty favorable magnetic control, are possible.

Impedance-interface waveguides based on metamaterials are interesting for their potential in wireless systems, sensors, and light-matter interaction, but applications based on these novel waveguides have been thwarted due to the technical difficulty of implanting such structures into existing systems. This study uses adiabatic mode-matching techniques to realize broadband efficient coupling between conventional microwave transmission lines and one-dimensional impedance-interface waveguides. This approach should facilitate photonic applications from microwave to optical frequencies.

This study proposes a comprehensive theory for the propagation of structured electron beams along atomic columns in crystals, by means of classical multislice and reformulated Bloch-wave algorithms, in the context of transmission electron microscopy. In particular, by making use of the symmetries of Bessel beams and Gaussian beams, the authors offer a clear description of the oscillatory behavior of the electron probe, including the dephasing of such an oscillation. This result provides a fresh methodological approach for the study of electron channeling, for the engineering of any desired depth behavior within a crystal, based on the initial probe shape.

There is a great drive for quantum technologies built on topological platforms. Generally these devices are very sensitive to temperature, due to the fragile nature of quantum states, and thus the management of heat becomes crucial. The authors aim to exploit the the peculiar structure of Andreev bound states in a two-dimensional topological Josephson junction. Using a normal-metal probe tunnel-coupled to the junction, a thermal rectification coefficient of up to 145% is attained. Furthermore, as the thermal transport here can be controlled by a small magnetic field, the rectification properties can greatly enhanced below the critical temperature.

Mechanical metamaterials exhibiting negative Poisson’s ratio (expanding, rather than contracting, in the direction perpendicular to an applied force) typically do not retain the ability to behave as typical materials with positive Poisson’s ratio, which restricts their application where adaptive behavior is desirable. This work demonstrates dynamic, on-demand regulation of auxetic behaviors in mechanical metamaterials, through a combination of structural design and multimaterial three-dimensional printing. The design principle provides a further degree of freedom for metamaterials exhibiting extraordinary and programmable mechanical behaviors.

Self-assembly of colloids into superstructures is very important for fabricating functional devices. This study demonstrates that in a colloidal system of anisotropic liquid-crystal droplets dispersed in isotropic glycerol, the droplets self-assemble into periodic superstructures in cells with patterned indium tin oxide (ITO) electrodes, due to the different surface tensions of the two components on ITO and glass. This self-assembly of the liquid-crystal droplets may be utilized to manufacture $e.g.$ switchable photonic crystals, sensors, or microlens arrays.

Magnonics, conventionally a room-temperature discipline, develops exceptional features when operated at cryogenic temperatures. The related low-temperature magnetization dynamics and their interplay with superconductors is of increasing interest for various avenues in data processing. This investigation of cryogenic magnonics reports the magnetization dynamics of a single-crystalline ultrathin film of yttrium iron garnet with a superconducting planar waveguide on top. The magnetic anisotropies shaped by the metal structures, drastically increased magnetic remanence, and magnetization dynamics by hybridization with a superconductor are discussed.

To study topological phases, break out your soldering iron… The valley Chern effect is interesting because robust interfacial modes can be created in a honeycomb lattice by a simple breaking of the system’s inversion symmetry. The effect has been emulated on a number of classical platforms, including mechanical and acoustic systems, but not on an electromagnetic one. This work proposes a modular setup of inductively coupled $LC$ resonators and shows how it can reproduce the valley Chern effect. Using realistic values for the electronic components, these circuits display extremely high $Q$ factors, making them ideal for exploring topological phenomena.

What is that noise? The authors discuss the switching to the resistive state of a current-biased Josephson junction affected by non-Gaussian (i.e. Lévy) stochastic fluctuations. Their theoretical study reveals the strong dependence of the features of switching-current distributions on peculiar parameters of the noise statistic. This phenomenon points the way to direct experimental investigation of $\alpha$-stable Lévy noise features, or to determine the Lévy component properties of an unknown noise source, for $e.g.$ improved telecommunication, or fault diagnosis in engineering, never mind the titular superconducting electronics.

Measurement and control of a microscopic system’s temperature distribution are increasingly important in diverse applications, yet measuring a three-dimensional (3D) temperature distribution at the microscale has remained out of reach. Here researchers use quantitative phase imaging to measure microscopically the 3D refractive-index distribution of a medium, from which its 3D temperature distribution is retrieved. This approach can facilitate our understanding of various thermal phenomena and applications spanning nanotechnology, bioengineering, and the semiconductor industry.

Quantum information helping condensed matter research: A dynamical quantum phase transition can occur during the time evolution of a suddenly quenched quantum system, in analogy to the nonanalyticity of the free-energy density at the critical temperature of a macroscopic system. The authors succeed in observing such a transition in a quantum simulation of the quench dynamics of a many-body system. This computational experiment shows that quantum phase transitions of many-body systems can be simulated successfully using a single superconducting qubit, by varying the control parameter over the range of momenta as the qubit’s spin-1/2 state evolves on the Bloch sphere.

Nonlinear phenomena occur frequently in nature, and are inherent to many optical systems. Though nonlinearities are often avoided or suppressed in optical resonators, here the authors exploit the nonlinear dynamics in a fiber-feedback optical parametric oscillator to realize an all-optical modulator based on period doubling. They find that this modulator is suitable for high-speed, low-noise stimulated Raman scattering imaging, with performance equal to that using conventional acousto-optical modulation, but without the associated hassles. Among other applications, this approach facilitates label-free imaging of biological samples.

To fabricate high-$T_c$ Josephson junctions, YBa${}_2$Cu${}_3$O${}_7$ (YBCO) bicrystal grain-boundary junction technology is used most frequently, as it is simple and yields junctions of reasonable quality. This technique only allows placement of junctions along a single line on a chip, though. The authors instead use a focused helium-ion beam (He-FIB) to “draw” Josephson junctions anywhere on the chip. High FIB doses can also produce highly resistive walls that can be used as circuit boundaries, avoiding extra lithography. A SQUID drawn by He-FIB on a YBCO bridge is presented as proof of principle. These results should enable development of more complex YBCO circuits at sub-$\mu$m scale.

The current generation of experimental quantum annealers can shed light on the effects of noise and thermalization present in superconducting quantum circuits. Recent advances in this technology allow for direct probing of intermediate dynamics during the annealing process. By pausing the annealing over spans ranging from micro- to milliseconds and allowing the system to equilibrate, the authors identify several regimes of interest, characterized by the time scales of the dominant processes. It is shown that alternative annealing schedules inspired by physics may prove a fruitful avenue for designing more efficient solvers for optimization problems in quantum computing.

Silicon photonics offers a mature platform for the fabrication of large-scale photonic circuits, with the potential for on-chip integration of electronics and detectors. Thus Si is an appealing basis for photonic quantum information processing, which will likely require chips to operate at few-K temperatures. This study determines the Kerr nonlinearity and two-photon absorption as a function of temperature in a silicon waveguide, and examines how this dependence would affect the generation of photon pairs. A moderately improved nonlinear figure of merit at low temperatures suggests improved heralding efficiency of on-chip parametric photon-pair sources.

Radiography is a powerful imaging technique that has found widespread use in medicine, security, and inspection of industrial processes. The technique is typically performed using either neutrons or photons, but neither of these probes is universally applicable. The authors experimentally realize a significant improvement in the contrast for small elemental variations in an object’s composition, by using both neutrons and photons simultaneously in transmission radiography. They present a multiparticle spectroscopic transmission radiography (MPSTR) technique that merges the information obtained from fast-neutron and photon probes produced by deuteron-based nuclear reactions.

Acoustic topological insulators can yield directional antennas for sound, enabling energy-efficient communication. This study explores using the interface-dependent spin Hall or valley Hall -like transport in a Kekulé lattice to generate directed beams: one, two in symmetric or antisymmetric fashion, or multiple beams. This topological refractive system could serve as an acoustic beam splitter, offering the advantages of stability, efficiency, and parity control.

Quantum computing hardware is now surpassing the 50-qubit level, which cannot be simulated with a classical computer. In practical use, generating the ground states of slightly different Hamiltonians ($e.g.$ induced by the different atomic coordinates in a molecule) is often required, but here the traditional form of the variational quantum eigensolver is inefficient. Thus the authors discuss “training” a quantum circuit with a small number of Hamiltonians and then generalizing the output by interpolation, and also propose a purpose-built quantum circuit, to greatly reduce the time needed to find ground states with a near-term quantum computer.

Wide-band stealth technology is important for many wave-based applications, particularly involving radar, but is held back because expanding bandwidth for radar cross-section reduction is extremely difficult for traditional phase-cancellation methods. This study develops a physical mechanism for multiwave destructive interference (MWDI), and presents a metasurface with variable layer thickness and engineered defect configuration. This unconventional metamaterial harnesses second destructive interference to break the current bandwidth constraints in radar stealth applications.

Thermal metamaterials have become an interesting playground for physicists in the quest to control the flow of heat. Studies so far have analyzed Fourier transfer of heat, which yields unphysical instantaneous propagation speeds. The authors instead use the scattering-cancellation technique to derive the invisibility condition for Maxwell-Cattaneo waves with harmonic time dependence of temperature fields. The physical properties of cloaking shells and device performance are studied. Subjects from medical tissue imaging to heat-assisted magnetic recording could benefit from these findings, and the design here may also be simpler to fabricate than transformation-acoustics prototypes.

The electrical control of vortex polarity and chirality is in high demand for spintronic oscillators, memory, and computing. Previous studies have shown that vortex polarity can be efficiently switched by electrical current, while chirality is not. Here the authors report that the dynamic transformation of edge solitons is able to switch both vortex polarity $and$ $chirality$, at low current and high speed. This work not only provides insight into the fundamental aspects of magnetic edge solitons, but also offers an efficient means to control vortex properties for low-power memory and computing applications.

Investigating the physical mechanisms of interaction between acoustic waves and fluids is significant for the detection of the properties of liquids. Researchers in this field have mainly used shear-mode acoustic wave sensors to measure fluid density and viscosity, but not the bulk modulus. Using an immersed gigahertz-range microsensor, one can obtain compressional information about a liquid from the observed propagation of longitudinal acoustic waves in the nonreflective regime. This work fills a gap by enabling measurement of a liquid’s bulk modulus in a microfluidic setting, with an eye toward physiological fluids in particular.

Calculating the energy spectra of molecules is a key computational problem, and one where a quantum computer can shine. For present-day quantum computers, short quantum algorithms that finish within the coherence time of the system must be designed. Thus the authors present a set of gates tailored to the problem at hand, which can be directly implemented in hardware. Experiments show that exchange-type gates that conserve the number of excitations are ideally suited for calculations in quantum chemistry. The team determines the energy spectrum of molecular hydrogen using a variational quantum eigensolver, plus a method from computational chemistry to compute the excited states.

Spintronic neurons that emit sharp voltage spikes are needed for neural-network hardware, for fast data processing with low power consumption. Conventional spintronic neurons, $i.e.$ spin-torque nano-oscillators (STNOs), emit sinusoidal waveforms, though. The authors suggest how to imitate neuron spiking in STNOs in which both magnetic layers are free and thin enough to be switched by current. Simulations show that the chaotic behavior of these frequency-tunable devices can be adjusted by the magnetic stack and current. The proposed spintronic relaxation oscillator is a promising building block for hardware neuromorphic chips, leveraging nonlinear dynamics for computing.

Superconducting quantum dots (SQDs), as generalized and tunable Josephson junctions, have potential for versatile components in electronic circuits and detectors. A key factor holding back their application is characterization, which often requires lengthy, impractical numerical methods. The authors offer simple analytical formulas that allow fast, reliable characterization of SQDs in different regimes, plus a straightforward and general method to locate the quantum phase transition from finite-temperature experimental data. These tools will dramatically speed up the characterization of real devices, and provide a handy framework to predict and refine the properties of these devices.